Btw, it is KAHLIL Gibran.
The book was quite popular with the Sixties generation, hence, it was quite popular with proponents of the Human Potential movement. CEDU wasn't the only organization exploiting its popularity and plasticity (interpretation open to misuse). Joe Gauld made good use of it as well (nowhere near as integrally as CEDU did, however)...as soapy quotes to prove his points, or to add palatability or credibility to his books and speeches re. Hyde School.
Another text that was also popular, especially amongst those inclined towards "Christian behavior modification," was Why Am I Afraid to Tell You Who I Am?: Insights Into Personal Growth, by John Powell. Not sure when that was first published, but it was before 1971 (latest edition appears to be 1995 or later).